I'd try it out in the hull first. Even with the smaller volume I think you will be okay. At 70% full, you have over 750ml of volume, and provided that the cylinder sits below the waterline, you need to displace about 700ml. How did I come up with that figure?
The upper half the of the Vanguard hull, moulded from polyester GRP weighs in at about 825 grams. The polyurethane resin parts- sail, front hydrovanes and rudder weigh about 120 grams. Now we know that 1000ml of water weighs 1000grams, metric makes it easy. But some materials are lighter than water, and some are heavier, so be careful when using weight alone to determine the size of a ballast tank.
Many plastics like ABS, polyurethane and polystyrene are the same density as water, so we can use weight to determine displacement. But composites like GRP are denser- the glass content is what makes the difference. Then we need to adjust our figures a little
In the case of the upper hull, we don't have all of it above the waterline, and a rough calculation removes about 120 grams and I'm erring on the conservative side. GRP laminate is about 25% denser than water, so when converting to displacement we reduce the volume still further.
825-120= 705 then 705x75%= 529 ml displacement for the upper hull
The polyurethane parts are equal in density to water, so in that case we can directly convert the weight to displacement, so 529+120= 649 ml displacement
That leaves another 51 grams- I haven't accounted for periscopes and paint, so that allows for that.